feat(download): smarter backfill — time-boxed chunks, run-until-done (backend)

Plan #693. Large-catalog backfill (Anduo) no longer sprints to the timeout
wall and dies as an error each run. Builds on the cursor checkpoint (#689).

- Time-boxed chunks: BACKFILL_TIMEOUT_SECONDS(1170)→BACKFILL_CHUNK_SECONDS(600),
  far under the 1350 soft limit. Hitting it = normal chunk boundary (the
  TimeoutExpired path already captures partial output + the cursor), not a
  near-wall death.
- Run-until-done state machine driven by config_overrides[_backfill_state]
  (running/complete/stalled). A running backfill auto-continues in chunks
  across ticks until gallery-dl exits cleanly (rc=0 = reached the bottom →
  'complete'); a safety-cap (BACKFILL_MAX_CHUNKS=200) + the #689 stall-guard
  pause a pathological walk as 'stalled'. Replaces the N-runs counter
  (backfill_runs_remaining repurposed as the cap countdown).
- Progress, not error: a chunk that timed out but advanced (cursor moved
  and/or files written) is reclassified TIMEOUT→PARTIAL (status 'ok').
- Retry storm tamed: gallery-dl retries 3→2, downloader timeout 120→60s, so
  one stuck CDN file fails in ~1-2 min not ~10 (Anduo #40838).
- API: POST /sources/{id}/backfill now takes {action: start|stop}; service
  start_backfill/stop_backfill; new enabled sources auto-arm run-until-done;
  source dict exposes backfill_state + backfill_chunks.

Frontend (Start/Stop control + state badge) lands in the next push.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

backend/app/tasks/download.py

@@ -31,7 +31,7 @@ _KEY_PATH = IMAGES_ROOT / "secrets" / "credential_key.b64"
 # SoftTimeLimitExceeded in-process, whereas the HARD limit SIGKILLs the
 # worker (no chance to finalize). Both gallery-dl subprocess budgets
 # (gallery_dl.py: _DEFAULT_GDL_TIMEOUT_SECONDS=870 tick,
-# BACKFILL_TIMEOUT_SECONDS=1170 backfill) MUST sit below the soft limit
+# BACKFILL_CHUNK_SECONDS=600 per backfill chunk, plan #693) MUST sit below the soft limit
 # so subprocess.run raises its own TimeoutExpired first — that path
 # captures partial stdout/stderr and finalizes the DownloadEvent. soft is
 # max-subprocess (1170) + ~180s phase-3 persist headroom; hard is soft +